This invention relates to x-ray target assemblies and radiation therapy systems and methods.
Radiation therapy involves delivering a high, curative dose of radiation to a tumor, while minimizing the dose delivered to surrounding healthy tissues and adjacent healthy organs. Therapeutic radiation doses may be supplied by a charged particle accelerator that is configured to generate a high-energy (e.g., several MeV) electron beam. The electron beam may be applied directly to one or more therapy sites on a patient, or it may be used to generate a photon (e.g., X-ray) beam, which is applied to the patient. An x-ray tube also may supply therapeutic photon radiation doses to a patient by directing a beam of electrons from a cathode to an anode formed from an x-ray generating material composition. The shape of the radiation beam at the therapy site may be controlled by discrete collimators of various shapes and sizes or by multiple leaves (or finger projections) of a multi-leaf collimator that are positioned to block selected portions of the radiation beam. The multiple leaves may be programmed to contain the radiation beam within the boundaries of the therapy site and, thereby, prevent healthy tissues and organs located beyond the boundaries of the therapy site from being exposed to the radiation beam.
X-ray bremsstrahlung radiation typically is produced by directing a charged particle beam (e.g., an electron beam) onto a solid target. X-rays are produced from the interaction between fast moving electrons and the atomic structure of the target. The intensity of x-ray radiation produced is a function of the atomic number of the x-ray generating material. In general, materials with a relatively high atomic number (i.e., so-called xe2x80x9chigh Zxe2x80x9d materials) are more efficient producers of x-ray radiation than materials having relatively low atomic numbers (i.e., xe2x80x9clow Zxe2x80x9d materials). However, many high Z materials have low melting points, making them generally unsuitable for use in an x-ray target assembly where a significant quantity of heat typically is generated by the x-ray generation process. Many low Z materials have good heat-handling characteristics, but are less efficient producers of x-ray radiation. Tungsten typically is used as an x-ray generating material because it has a relatively high atomic number (Z=74) and a relatively high melting point (3370xc2x0 C.).
The bremsstrahlung process produces x-rays within a broad, relatively uniform energy spectrum. Subsequent transmission of x-rays through an x-ray target material allows different x-ray energies to be absorbed preferentially. The high-Z targets typically used for multi-MeV radiation therapy systems produce virtually no low energy x-rays (below around 100 keV). The resultant high energy x-rays (mostly above 1 MeV) are very penetrating, a feature that is ideal for therapeutic treatment. In fact, in treatment applications, it is desirable not to have a significant amount of low energy x-rays in the treatment beam, as low-energy beams tend to cause surface burns at the high doses needed for therapy.
Before and/or after a dose of therapeutic radiation is delivered to a patient, a diagnostic x-ray image of the area to be treated typically is desired for verification and archiving purposes. The x-ray energies used for therapeutic treatment, however, typically are too high to provide high quality diagnostic images because high-energy therapeutic beams tend to pass through bone and tissue with little attenuation. As a result, very little structural contrast is captured in such images. In general, the x-ray energies that are useful for diagnostic imaging are around 100 keV and lower. High-Z targets produce virtually no x-rays in this diagnostic range. Low-Z targets (e.g., targets with atomic numbers of 30 or lower, such as aluminum, beryllium, carbon, and aluminum oxide targets), on the other hand, produce x-ray spectra that contain a fraction of low-energy x-rays that are in the 100 keV range and, therefore, are suitable for diagnostic imaging applications. See, for example, O. Z. Ostapiak et al., xe2x80x9cMegavoltage imaging with low Z targets: implementation and characterization of an investigational system,xe2x80x9d Med. Phys., 25 (10), 1910-1918 (October 1998). Because of the need for verification and documentation of therapeutic treatments, xe2x80x9cportal filmsxe2x80x9d or xe2x80x9cportal imagesxe2x80x9d typically are taken in real time (or nearly real time) using the high-energy x-ray treatment beam (see, e.g., U.S. Pat. Nos. 5,686,733, 4,995,068 and 5,138,647). If the images are collected electronically, various image enhancement techniques may be employed to enhance contrast and general quality (see, e.g., U.S. Pat. Nos. 5,675,624 and 6,148,060). One way to improve image quality is to use a separate low-energy diagnostic x-ray source. This source may produce a beam that is separated from the high-energy treatment beam and may be aimed in the opposite direction through the patient (see, e.g., U.S. Pat. No. 5,233,990). Alternatively, the diagnostic beam may be directed through the collimation system of the treatment beam (see, e.g., U.S. Pat. Nos. 5,471,516 and 6,134,295). In another approach, separate diagnostic and therapy devices are used, with careful registration and restriction of patient motion as the patient is transferred between each device (see, e.g., U.S. Pat. No. 5,851,182).
For sub-MeV diagnostic x-ray systems (as opposed to the multi-megavolt systems typically used for therapy), x-ray absorption edges advantageously may be used to enhance images. For example, dual-energy x-ray techniques may be used to separate bony tissue from soft tissue in medical imaging. Typically, the two distinct energy bands are selected to be above and below an absorption edge of the object to be imaged. By subtracting the image data produced with the higher energy x-ray radiation from the image data produced with the lower energy x-ray radiation, an enhanced contrast image may be obtained.
Many different dual-energy x-ray schemes have been proposed. In a switched mode dual-energy x-ray system, the voltage of an x-ray tube periodically is changed from a high voltage to a low voltage to shift the energy spectra of the resulting x-ray beams. A broadband detector collects image data produced by the two different x-ray radiation spectra. In an alternative approach, a broadband (or polychromatic) x-ray beam may illuminate an object, and a dual band detector may be used to collect image data at two different x-ray radiation energy bands. Typically, a front detector measures total x-ray flux and a rear detector measures high energy x-rays that pass through an intervening filter. High contrast x-ray images may be obtained from these two measurements. Still other dual energy x-ray imaging schemes have been proposed.
The invention features a multi-region target that is configured to selectively generate two different energy distributions when exposed to an excitation electron beam. In particular, the inventive multi-region target includes multiple regions with different x-ray generating characteristics. Thus, the interaction between an excitation electron beam and the target generates an x-ray beam with an energy distribution that depends upon which target region is exposed to the excitation electron beam. The different x-ray spectra may be used to produce an enhanced contrast x-ray image. The invention also features a novel method of detecting the rotational position of the multi-region target based upon the contrast level of the resulting images.
In one aspect, the invention features a target assembly comprising a multi-region target having an exposed surface, a first region and a second region, and a cooling mechanism coupled to the first and second regions of the target. The first region comprises a first x-ray generating characteristic. The second region is laterally displaced from the first region with respect to an excitation beam incident upon the exposed surface and comprises a second x-ray generating characteristic that is different from the first x-ray generating characteristic.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The first region preferably comprises a first x-ray generating material composition, and the second region preferably comprises a second x-ray generating material composition that is different from the first material composition. The first x-ray generating material composition preferably includes a material with a relatively high atomic number, and the second x-ray generating material composition preferably includes a material with a relatively low atomic number. The relatively high atomic number material may have an atomic number of 72 or higher, and the relatively low atomic number material may have an atomic number of 30 or lower. The relatively high atomic number material may be tungsten or tantalum or gold, and the relatively low atomic number material may be aluminum, beryllium, carbon or aluminum oxide.
In some embodiments, the first region is larger than the second region. The target may be substantially disk-shaped. The first and second regions may correspond to respective sectors of the substantially disk-shaped target. Alternatively, the second region may correspond to one or more peripheral portions of the substantially disk-shaped target, and the first region may correspond to remaining portions of the substantially disk-shaped target. In this case, the target assembly may be shifted laterally so that the electron beam strikes only the first region or strikes both regions as the target rotates.
The cooling mechanism may include a target holder configured to support the target for rotation and to expose the target to a cooling fluid stream, whereby the target rotates about an axis of rotation upon exposure to the cooling fluid stream.
In another aspect, the invention features a radiation therapy system comprising a source of an electron beam, and the above-described multi-region target positioned to intercept the electron beam.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
The radiation therapy system may include a computer configured to identify images produced from x-rays generated by the first target region and to identify images produced from x-rays generated by the second target region. The computer may be configured to identify images based upon image contrast. The computer may be configured to produce an enhanced image by combining image data produced from x-rays generated by the second target region with image data produced from x-rays generated by the first target region.
In another aspect, the invention features a radiation therapy method in accordance with which an electron beam is generated and the above-described multi-region target is positioned to intercept the electron beam.
Embodiments in accordance with this aspect of the invention may include one or more of the following features.
Images preferably are generated from x-rays generated by exposing the target to the electron beam. Images produced from x-rays generated by the first target region may be identified, and images produced from x-rays generated by the second target region may be identified. Images may be identified based upon image contrast. An enhanced image may be produced by combining image data produced from x-rays generated by the second target region with image data produced from x-rays generated by the first target region.
Among the advantages of the invention are the following.
The invention provides both a low-Z target to provide high-quality diagnostic images and a high-Z target to provide the bulk of the therapeutic treatment. Because much of the treatment dose is delivered with beam from the high-Z target, damage to skin and surface tissue is less than would be obtained from a low-Z target alone. In addition, the high-Z and low-Z targets may be interchanged rapidly, reducing the risk that an anatomical structure (e.g., surface tissue or internal organ) will move between the two exposure modes. This feature also enables two different exposure images to be combined to enhance image contrast by removing high-energy x-ray noise (or image degradation) from the resultant image. By applying the proper gain to a high-Z image and combining this data with image data obtained with the low-Z target, any noise or image artifacts due to the high energy x-rays (which are present with both targets) may be subtracted away, leaving the image that would have been obtained from a beam solely composed of lower-energy x-rays. In effect, this artificially synthesizes a low-energy x-ray spectrum with little high-energy x-ray content.
Other features and advantages of the invention will become apparent from the following description, including the drawings and the claims.